As described in the '124 application (and its incorporated references), in some radio bands, such as the 217 220 MHz VHF band, as a non-limiting example, governmental licensing agencies (e.g., the Federal Communications Commission (FCC)) customarily grant primary licensees non exclusive use of the band for a variety of communication services, such as push to talk voice transmission. Primary users pay for this licensed use with an expectation that they will not encounter interference by other users. The FCC also allows secondary users to access the same band and the same channels within the band on a ‘non-interfering’ or secondary basis, whereby a channel may be used by a secondary, non-licensed, user, so long as the primary user is not using that channel.
The FCC and similar agencies in foreign countries are continually looking for ways to allow the expanded use of these licensed radio frequency bands without reducing the quality of service available to primary users. For secondary users, these bands provide a cost free opportunity with excellent radio transmission properties for telemetry and other applications. Because secondary users must not interfere with primary users, however, complaints of interference from a primary user to the FCC may result in its issuing an administrative order requiring the secondary user to move to another portion of the band or leave the band entirely. Such a spectral transition is disruptive to the secondary user's service and can be expensive, especially if site visits, equipment modification, or exchange are required in order to implement the mandated change. It will be appreciated, therefore, that there is a need for a mechanism that allows a secondary user to employ a licensed band on a non interfering basis and that will adapt the radio's frequency usage should new primary users appear. It should be noted that primary users always have priority over secondary users and there is no first use channel frequency right for secondary users.
Advantageously, the invention described in the '753 application incorporated into the '124 application successfully addresses this need by means of a monitored spectral activity-based link utilization control mechanism, and the present application extends the range of the transceiver links through ‘repeatering’. Briefly reviewing this link utilization control mechanism, which may be used with a star-configured communication system among other topologies and configurations, such as that depicted in the reduced complexity diagram of FIG. 1, a spectral reuse transceiver installed at a master site 10 communicates with respective spectral reuse transceivers installed at a plurality of remote sites 12. Each spectral reuse transceiver operates with a selectively filtered form of frequency hopping for producing a sub-set of non interfering radio channels or ‘sub-channels’. It should be noted here that other configuration or network topologies may be used consistent with the invention disclosed herein. Thus the invention may be used with radio links between transceivers in other topologies, such as point-to-point, and individual links in mesh networks without limitation, consistent herewith.
For this purpose, the master site 10 periodically initiates a clear sub-channel assessment routine, in which the master site and each of the remote sites 12 participate, in order to compile or ‘harvest’ a list of non-interfering or ‘clear’ sub-channels (such as 6.25 KHz wide sub-channels), which may be used by participants of the network for conducting communication sessions that do not ostensibly interfere with any licensed user. By transmitting on only clear sub-channels, a respective site's spectral reuse transceiver is ensured that it will not interfere with any primary user of the band of interest.
Except when it is transmitting a message to the master site, each remote user site sequentially steps through and monitors a current list of clear sub-channels (that it has previously obtained from the master site), in accordance with a pseudo-random (PN) hopping sequence that is known a priori by all the users of the network, listening for a message that may be transmitted to it by the master site transceiver. During the preamble period of any message transmitted by the master site, each remote site's transceiver scans all frequency bins within a given spectrum for the presence of radio frequency (RF) energy. Any bin containing energy above a prescribed threshold is marked as a non-clear sub-channel, while the remaining sub-channels are identified as clear (and therefore available for reuse) sub-channels.
Whenever a remote site notices a change in its clear sub-channel assessment, it reports this to the master site at the first opportunity. As the master site has received clear sub-channel lists from all the remote sites, it logically combines all of the clear sub-channel lists, to produce a composite clear sub-channel list. This composite clear sub-channel list is stored in the master site's transceiver and is broadcast to all of the remote sites over a prescribed one of the clear sub-channels that is selected in accordance with a PN sequence through which clear sub-channels are selectively used among the users of the network. When the composite clear sub-channel list is received at a respective remote site it is stored in the site's transceiver.
To ensure that all communications among the users of the network are properly synchronized (in terms of the composite clear sub-channel list and the order through which the units traverse, or ‘hop’ through, the clear sub-channel entries of the clear sub-channel list), the master site's transceiver transmits an initialization message on an a priori established clear sub-channel, which each of the remote units monitors. This initialization message contains the clear sub-channel list, an identification of the preamble channel, a PN sequence tap list, and a PN seed that defines the initial sub-channel and hopping sequence for the duration of an upcoming transmit burst. Once a remote site has received an initialization message, that site will transition to normal multiple access mode.
This procedure is similar to that disclosed in the '124 application which is intended for use with a star-configured communication system such as that depicted in the reduced complexity diagram of FIG. 1 in which a spectral reuse transceiver installed at a master site 10 communicates with respective spectral reuse transceivers installed at a plurality of remote sites 12. Each spectral reuse transceiver operates with a selectively filtered form of frequency hopping for producing a sub-set of non interfering radio channels or ‘sub-channels’.
For further details of the architecture and operation of the spectral reuse link control mechanism disclosed in the above-referenced '753 and '124 applications, attention may be directed to those documents. They will not be detailed here in order to focus the present description on the problem of simplex and duplex ‘repeater’ embodiments, whereby one or more remote site transceivers operate in repeater mode to extend the range of the network.